Generation of efficient packaging cell lines for lentiviral vectors is hampered by the cytotoxicity of some of the products of the gag and pol genes. Thus, it is desirable to have inducible expression of gag and pol so that optimal clones that will express gag and pol at high levels when needed can be selected in the absence of gag/pol expression.
A method for selecting cells which express gag and pol and thus are useful as packaging cells is obtained by linking a selectable marker to the gag/pol expression cassette of a packaging vector in such a way that the marker is expressed by the same promoter which controls expression of the gag/pol genes although expression of the gag/pol genes is suppressed. Efficient expression of the marker predicts efficient expression of the gag/pol genes on induction.